1. Field
At least one embodiment relates to an image forming apparatus including a plurality of developing devices that use the electrophotographic scheme.
2. Description of the Related Art
Generally, an electrophotographic printer forms an electrostatic latent image by scanning light onto a photoconductive drum charged with a predetermined potential, develops the electrostatic latent image with toners of predetermined colors, and transfers and fixes the developed image onto a sheet of paper, such that a color image is formed.
In order to print a full-color image, the colors yellow (Y), magenta (M), cyan (C), and black (K) are required for the image forming apparatus. Thus, four developing devices are required to fix toners of four colors onto an electrostatic latent image.
A high voltage (e.g., hundreds of volts or thousands of volts), such as a developing bias voltage applied to the developing roller so as to fix the toner of the developing roller to a photoconductive drum or a supply bias voltage applied to the supply roller so as to provide the toner to a developing roller, is applied to each developing device.
A color image-forming scheme is classified into a multi-pass scheme and a single-pass scheme. The multi-pass scheme forms a color image by rotating one photoconductive drum several times. The single-pass scheme forms a color image by rotating each of the photoconductive drums only once.
In the case of the image forming apparatus based on the multi-pass scheme, four developing devices are sequentially operated, so that a high voltage is also sequentially applied to the four developing devices. In this case, a predetermined voltage is applied to a developing roller of each developing device and a supply roller in such a manner that a predetermined potential difference is formed between the developing roller and the supply roller.
In order to form a difference in potential between the developing bias voltage and the supply bias voltage, each developing device uses Zener diode components.
However, these Zener diode components are generally installed at the last end serving as an output side that provides the developing roller and the supply roller of each developing device, so that it is impossible to adjust a deviation of components related to the zener voltage of the Zener diode.
For example, if the source voltage provided to each developing device is adjusted to provide a voltage (in which a deviation of components of the Zener diode corresponding to the yellow (Y) developing device is considered) to a yellow (Y) developing device, a deviation of several tens of volts occurs in voltage applied to other developing devices, so that it is difficult for each developing device to control a developing voltage level at a desired voltage level. In other words, due to a deviation of Zener diode components that form a difference in potential between the developing bias voltage and the supply bias voltage, it is difficult for individual developing devices to acquire their desired color images, resulting in a deterioration of the color image quality.